Thread: Accessing an object vs. accessing a reference to the object

  1. #1
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    Dec 2006
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    Accessing an object vs. accessing a reference to the object

    Everything seems to compile fine, but I'm getting a run-time error. What I don't understand is these are identical, the only difference being one is accessing an object directly and the other is accessing a reference to the object. For some reason I can declare the reference no problem, but as soon as I try to access it, it breaks.
    Code:
    //part of cEntity.cpp
    void cEntity::SetWornHead(cArmor * head)
    {wornhead = head;}//wornhead declaration: cArmor * wornhead
    
    cArmor* cEntity::GetWornHead(void)
    {return wornhead;} 
    
    void cEntity::SetWornShoulders(cArmor shoulders)
    {wornshoulders = shoulders;} //wornshoulder declaration: cArmor wornshoulders
    
    cArmor cEntity::GetWornShoulders(void)
    {return wornshoulders;}
    
    //constructor
    cEntity::cEntity(char* thisname)
    {
         SetWornHead(&gArmorTravelersHead);
         SetWornShoulders(gArmorTravelersShoulders) 
         GetWornHead()->GetArmorClass(); //threw that in just to see if it would work in the constructor, and it did.
    }
    Everything up to here works, butwhen i want to do something like this, I get the error
    Code:
    void cEntity::SetAC(void)
    {
    	ac = GetWornHead()->GetArmorClass() + //doesnt work
    		GetWornShoulders().GetArmorClass();//works
                    //GetArmorClass() is a member of the class cArmor
    }

  2. #2
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    when gWornArmorTravelersHead falls out of scope wornhead is pointing to undefined behavior. I don't see any reason that wornhead needs to be a pointer.

    Prefixing an & to a variable returns the address, it does not turn it into a reference. declaring a type with the & creates a reference variable.
    Code:
    cArmor & wornhead = helm;
    makes any changes to wornhead change helm, realistically wornhead is just another name for helm and if at all possible the compiler will just treat it as such.
    Code:
    trivial a;
    trivial &b = a;
    std::cout << a <<", " << b << std::endl;
    could legally, and probably should generate exactly the same code as
    Code:
    trivial a;
    std::cout << a <<", " << a << std::endl;
    The compiler usually can't quite do this, so it fakes it by using a pointer behind the scenes but it's a little bit more than just a "pointer that can never be null".

  3. #3
    Hurry Slowly vart's Avatar
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    Rishon LeZion, Israel
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    You don't have a reference in your code...
    SetWornShoulders copies the object to your class member.
    SetWornHead sets the pointer to the external object. And as it was said - in case that the object is destroyed by some reason you have a dangling pointer you cannot access.

    Note that the SetWornShoulders can be rewritten in the following way:
    Code:
    void cEntity::SetWornShoulders(const cArmor& shoulders)
    {wornshoulders = shoulders;}
    The logic stays the same, but instead of copying object twice - once in the temporary variable put into function and second time into your member variable, - you avoid first copy and leave only the second one
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  4. #4
    Registered User
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    Dec 2006
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    9
    thx for the replies....for some reason I'm having a hard time getting a good grasp of referrers/pointers etc, most everything else has come w/ relative ease to me and I have been able to get it by just reading/using. Does anyone know of some good material on this subject specifically?(either excercises or reading material or anything really would be great).

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